These findings are important for worldwide understanding of fire’s impact on the carbon cycle and for modeling the future of global carbon and climate change. The results offer a new perspective on the impact of fire on soil fertility.

“Almost all the synthesis studies done to date conclude that fire has relatively little effect on soils, but in large part, researchers focused on a single fire event,” says Adam Pellegrini, a postdoctoral scholar at the School of Earth, Energy & Environmental Sciences at Stanford University who is also lead author of a new paper outlining the findings.

Instead, the researchers examined soil carbon and fertility in different ecosystems over 65 years.

“We are interested in the ways ecosystems change over many decades,” says Rob Jackson, professor of Earth system science and senior author of the study. “As the Earth’s climate warms, we need to understand how changing fire frequencies alter soils and plant growth.”

Pellegrini, Jackson, and their colleagues found that estimates of fire impacts on the ecosystem’s ability to store carbon may be substantially off. The new findings could help land managers better weigh how often they use prescribed burns to manage grasslands and forests.

Fire and soil

Focusing on three different types of landscapes—savanna grasslands, broadleaf forests, and needleleaf forests—from 48 sites covering multiple continents, the researchers compiled records of soil fertility after fires over up to 65 years.

Comparing the changes in soil nutrients over time, they found that in frequently burned areas in savannas and broadleaf forests, there was a 36 percent reduction in soil carbon and a 38 percent reduction in nitrogen compared to areas that were protected from fire. Conifer forests did not show this reduction in soil carbon and nitrogen after fires.

“You can imagine after only one year, fire wouldn’t make much of a difference, but over multiple years and repeated burning, the lack of plant inputs into soils decreases carbon and nutrients,” says Pellegrini.

“Plant productivity is what draws carbon out of the atmosphere—that’s what sequesters the carbon. What we see over time is not just a net increase in the carbon emissions to the atmosphere, but a net decrease in carbon sequestration,” he says.

After the team found that repeated fire causes the continued loss of carbon and nitrogen in savannas and broadleaf forests, they wanted to see how it might affect plant recovery. They used a vegetation model to predict global plant growth in frequently burned areas and found the loss of nitrogen in soils suppressed the regrowth of plants.

Carbon storage and climate change

The slower plant growth has implications for carbon storage, and Jackson warned there’s a long-term feedback that should be considered.

“There are additional losses and changes in carbon that can happen decades later because plants are unable to grow back as quickly as they would have,” says Jackson, who is also a senior fellow at the Woods Institute for the Environment.

Surprisingly, needleleaf forests didn’t show the same loss of nutrients. Instead, the conifer-dominated forest soils actually had more carbon and nitrogen present with frequent burning. Pellegrini explains that burning of the dense layer of partially decomposed pine needles on the forest floor might provide ash to the soils, and adds that future work will further examine the effects of fire in this type of forest.

The researchers stress that they are not advocating fire suppression. “Fires often increase the diversity of plants and reduce the risk that a landscape will have a high-intensity fire,” says Pellegrini.

Adam Pellegrini is also a NOAA Climate and Global Change Postdoctoral Fellow. Rob Jackson is also a senior fellow at the Precourt Institute for Energy. The paper also includes additional authors from Stanford University; Lund University; the University of Minnesota; Western Sydney University; Yale University; the University of Wisconsin–Stevens Point; Kansas State University; the University of Utah; and the University of California, Irvine.

The National Oceanic and Atmospheric Administration, the Gordon and Betty Moore Foundation, and the Department of Energy funded the study.